clarify comments

And throw instead of return null for invalid settings.
diff --git a/include/json/writer.h b/include/json/writer.h
index 9f51d07..36ed4b9 100644
--- a/include/json/writer.h
+++ b/include/json/writer.h
@@ -92,9 +92,9 @@
     Available settings (case-sensitive):
     - "commentStyle": "None" or "All"
     - "indentation":  "<anything>"
-    - "enableYAMLCompatibility": False or True
+    - "enableYAMLCompatibility": false or true
       - slightly change the whitespace around colons
-    - "dropNullPlaceholders": False or True
+    - "dropNullPlaceholders": false or true
       - Drop the "null" string from the writer's output for nullValues.
         Strictly speaking, this is not valid JSON. But when the output is being
         fed to a browser's Javascript, it makes for smaller output and the